The overall goal is to characterize the role that body fat reserves and hypothalamic peptide mechanisms, especially neuropeptide Y (NPY), play in controlling daily torpor. Daily torpor is an extreme form of temperature regulation that evolved to help animals contend with limited food availability by uniquely reducing energy expenditure. The central and peripheral neural mechanisms integrating overall energy balance and torpor will be explored. Annual rhythms of food intake and body mass, reproduction, and daily torpor are well documented in the Siberian hamster (Phodopus sungorus), the model species in all experiments. Specific aims include: 1) contrasting the effects of direct intra-medial preoptic area (MPOA) injections of NPY on body temperature, specifically its ability to induce torpor and affect the timing of changes in brown adipose tissue temperature vs. body temperature;2) developing an anatomical understanding of the distribution of NPY receptor subtypes in the Siberian hamster hypothalamus;3) comparing and contrasting NPY receptor agonist activation and antagonist inhibition of NPY receptor sites in the MPOA on torpor induction;4) evaluating the necessity of the MPOA for both induced (NPY &2DG) and natural torpor using neuron-specific neurotoxic lesions;5) comparing and contrasting effects of direct injections of NPY and NPY receptor agonists into the paraventricular nucleus of the hypothalamus on interscapular brown adipose tissue temperature (and body temperature).